In a typical tubular key there is an index element, which may an outwardly extending finger adjacent the open end of the barrel, and a series of notches arranged at predetermined minimal angular distances from the index element around the outer periphery of the barrel, and which extend for predetermined length from the barrel end. The arrangement of the notches determines whether the key will fit a particular lock, and the width, depth and length of the notches are critical, as is their angular spacing from one another. Virtually zero tolerances are essential, and since the duplicating machine must be easy to adjust and operate by relatively unskilled persons, it must be extremely simple, rugged and not prone to get out of adjustment.
While tubular key duplicating machines have previously been devised, so far as is known they have entailed the interposition of some sort of mechanism or moving parts between the original key and the blank. Any such mechanism either complicates the machine or militates against complete accuracy, or both. In the Simon patent (supra) for example, the holder for an original key is coupled to the holder of the blank via a gear train so that the fidelity with which the blank follows the rotating movements of the original depends upon the amount of backlash in the gear train, bearing tolerances and accuracy. In other machines, the key blank chuck is angularly indexed from one notching-position to another which are selected by the operator, sometimes according to a code, so that it is possible to cut notches in the blank which do not correspond to those in the original key.